Abstract

An experimental technique for evaluating the activity of chromite in
multicomponent spinels was developed by equilibrating the spinel of interest with a Pt-alloy
under a controlled temperature and oxygen fugacity. The thermodynamic
properties of the ternary Pt-Fe-Cr system was evaluated, such that activities of Cr and Fe
in an equilibrated Pt-alloy can be used to calculate the activity of chromite in the spinel of
interest.

The ternary activity model formulation used is based on the characterization of
each bounding binary system, Pt-Fe, Pt-Cr, Fe-Cr, with the addition of ternary interaction
terms. The Pt-Fe and Pt-Cr systems are described as asymmetric regular solutions with
interaction parameters of W_(PtFe)=-138.0±3.3, W_(FePt)=-90.8±24.0, and W_(PtCr)=-129.1±1.2,
W_(CrPt)=-80.9±4.4, and D_(PtCr)=+94.4±2.5 kJ/mol (1σ), respectively. Combined with
literature thermodynamic properties for the Fe-Cr system, the ternary interaction
parameters in the Pt-Fe-Cr system were found to be C_(Cr)=0, C_(Pt)=+ll5.7, and C_(Fe)=-68.6
kJ/mol.

Using this technique, the metamorphic history of equilibrated ordinary chondrites
was evaluated by examining the compositions and textures of olivines, pyroxenes,
spinels, and alloys. Equilibrium temperatures based on Fe-Mg exchange between olivine
and spinel exhibit a range of 680-796°C in H, L, and LL ordinary chondrites spanning
petrographic type 4 to 6. Type 4 chondrites in all groups record variable temperatures
that are lower than or equal to those of types 5 and 6 chondrites, implying decoupling of
metamorphic temperature from petrographic type. Cooling rates near 800°C were found to be 1-3 K/Ma, slow enough to allow continous re-equilibration of spinel grains from
peak metamorphic temperatures to the olivine-spinel equilibration temperature.

The temperature-oxygen fugacity relationships in equilibrated H chondrites were
constrained from spinel-alloy and olivine-pyroxene-alloy phase assemblages based on the
mineral compositions and activity-composition models. Log_(10)f_(02) values based on the
assemblage olivine-pyroxene-alloy are -1.75±0.02 log units below Iron-Wüstite (IW)
buffer, regardless of petrographic type. The log_(10)f_(02) values calculated based on the
spinel-alloy coexistence are at least ~1.5 log units more oxidizing than those based on the
olivine-pyroxene-alloy if olivine-spinel equilibration temperatures (728-820°C) are
assumed. This probably indicates that closure for spinel-alloy equilibria occurred under
retrograde conditions at temperatures below 700°C.